1. Field of the Invention
This invention relates to a system for sensing the position of a ferromagnetic object, such as, for example, a tooth on a moving gearwheel and a magnetoresistive transducer arrangement for determining such position as well as the use of such sensed information to determine the speed and/or position of the magnetic object. This invention is particularly suited to various speed and/or position measurement requirements found on automotive vehicles.
2. Brief Description of the Prior Art
The position and/or velocity of a ferromagnetic object can be magnetically sensed by measurement of the change in a magnetic field caused by the movement of the ferromagnetic object through the magnetic field. The sensor utilized in such measurement arrangements generally comprises a transducer, a conditioner and packaging.
The use of transducers to measure changes in a magnetic field is well known. Such transducers convert some physical phenomenon into an electrical signal from which the measurement is derived. Typically, such transducers have been variable reluctance sensors wherein the movement of a ferromagnetic object in the vicinity of and through the magnetic field of a coil/permanent magnet thereof causes a voltage to be induced in the coil due to the change of the magnetic flux pattern through the coil.
Magnetoresistive transducers are also known in the art and have also been used in the past in conjunction with other circuitry to indicate position and/or measure velocity of a magnetic object, an example of which is set forth in U.S. Pat. No. 4,754,221. There are several known materials which display magnetoresistive properties, the most common such material being permalloy, a well known nickel-iron alloy. Films of these materials are sensitive to magnetic fields along the thin plane of the material rather than through their thickness dimension. Typical prior art in this regard is set forth in the patent of Kuijk No. 4,052,748 and in an article entitled Magnetic Field Sensors Using the Magnetoresistive Effect by U. Dibbern, Sensors and Actuators, 10 (1986) 127-140. Indium antimonide, a semiconductor material, is another of the materials which exhibits magnetoresistive properties, this material, however, being sensitive to magnetic fields through its thickness dimension rather than along the thin plane of the material as in the case of permalloy. The discussion herein will be confined to the permalloy type of magnetoresistive element.
In the formation of magnetoresistive elements from permalloy, the permalloy is normally sputtered as a thin layer or film onto a substrate, for example silicon, the permalloy film residing on the substrate and having a thickness of 20 to 200 nanometers, preferably about 50 nanometers. The permalloy is further processed, either during or after sputtering, to induce a permanent magnetization in the film. The permalloy film is often used in a configuration in which a full or half wheatstone bridge pattern is etched onto the film such that a voltage divider network is created. Resistance changes in the film are thereby converted to a voltage output. Permalloy magnetoresistive transducers respond to changes in the magnetic field vector in the plane of the film and do not respond to magnetic field components normal thereto. This response is represented by a change in the electrical resistance of the permalloy as a function of the angle between the magnetic field vector passing therethrough in the plane thereof and an electric current vector in the same plane.
The prior art systems which use the variable reluctance sensors suffer from the limitations that, at low speeds, the output voltage thereof becomes small and more difficult to use and that the sensor has a strong dependence upon the gap between the target and the face of the sensor.
The transducer response is elicited when an external field component in the plane of the film causes the internal magnetizationof the film to rotate. Fields in the plane of the film can be considered to have two orthogonal components, one parallel to the initial magnetization called the bias field and one perpendicular to the initial magnetization called the measurand field. The bias field is required to ensure that the initial direction of magnetization in the film is stable under the influence of external magnetic disturbances. Furthermore, the sensitivity of the transducer to the measurand field is dependent upon the magnitude of the bias field. The bias field should therefore be as uniform and as constant over the entire film surface as possible. The prior art (shown in FIG. 2) uses a permanent magnet with magnetization at an angle to the magnetoresistive surface to provide the bias field. Such a bias field generation suffers from the drawback that the field lines on one side of the axis of the magnet tend to return in a direction in opposition to the desired bias field, resulting in bias field nonuniformity and sensitivity to external influences (such as nearby ferromagnetic objects).
Furthermore, it is desired that the transducer bridge be calibratable upon assembly to compensate for variations in the magnetoresistive element and magnet characteristics. Such calibration should allow the transducer bridge output voltage to be adjusted to a desired level in order to obtain optimum function of the transducer in concert with associated electronic signal conditioning apparatus. The prior art does not recognize this problem in that it neither makes mention of the need for calibration nor the means to accomplish calibration.